1. Field of the Invention
In recent years, the use of aircraft as a transportation and recreation medium has become extremely widespread, resulting in a tremendous growth of private, commercial and military aircraft. As the number of aircraft has increased, the probability of mid-air collisions has also increased. Several methods are being advanced to automatically prevent mid-air collisions, principally by the use of various radio communication techniques. Some of the methods rely on direct radio communications between the involved aircraft. Other methods utilize ground-based control systems and send maneuver commands to the involved aircraft which theoretically resolve the potential conflict. Both the air-derived and ground-derived collision avoidance systems require that accurate information pertaining to aircraft altitude be transmitted by some form of radio communication. However, presently available barometric encoding altimeters are both expensive and unreliable.
2. Description of the Prior Art
FIG. 1 is a schematic of a prior art conventional altimeter. The altimeter is driven by an aneroid bellows 10 as the ambient air pressure (static pressure) on the outside of the bellows increases or decreases. As the aircraft climbs to a higher altitude, the spring constant of the bellows material causes the bellows to extend, driving the linkage 12 and gears 13. The gears 13 are connected to needles 14 on dial 17 and drive the needles to provide a visual display of altitude to the pilot. Also connected to the gearing is an encoder 15 which provides electrical signals to other equipment. Temperature compensating means 16 are also provided around the bellows 10 to keep the accuracy of the instrument within tolerance as the ambient temperature varies and the mechanical properties of the materials change incident thereto. The compensation is generally embodied in some form of bimetallic spring.
The fundamental problem with the conventional type altimeter is that all the energy to drive the gears, indicating hands and encoder must come from the mechanical energy stored in the spring material of the aneroid bellows working against the static air pressure acting on the end of the bellows. The forces and motions involved are so small that precise, low friction mechanisms are a necessity. Also required are great care and skill in the assembly, adjustment and calibration of the instruments. The minuteness of the available forces mitigates against using mechanical contacts for the encoding function but rather require relatively high cost precision electro-optical position encoders. This in turn leads to a costly and fragile instrument.
It will be further appreciated that the accuracy of the instrument depends upon the mechanical characteristics of the bellows 10 as it extends and contracts under the influence of the changing static air pressure. Any friction or binding of the mechanism will cause errors because of the very small forces available. Also, any change in the mechanical characteristics of the bellows due to, for instance, temperature change will also cause errors unless properly compensated for.